The kinematic system of coordinate measuring machines, tool machines and numerous other processing machines has a probe or a tool movable in space or in a plane. When higher requirements as to precision are imposed, two or three linear guides are realized which are built up on each other and are at right angles to each other. Linear measuring systems are provided for the linear guides. These known systems have the advantage of a simple kinematic which can be realized, however, only with very complex constructive measures and high manufacturing cost. In addition, the moved mechanical components of these systems have a relatively high mass.
Planar kinematic systems having two parallel pivot linkages can be realized in a structurally more simple manner. Coordinate measuring apparatus built according to this principle and having probes movably guided at one linkage arm have two parallel pivot axes one behind the other in one plane. These coordinate measuring apparatus are described, for example, in British Patent 1,498,009 and in the following U.S. Patents: U.S. Pat. Nos. 4,891,889 and 5,396,712. The linkage arm of the coordinate measuring apparatus described in U.S. Pat. No. 5,396,712 is additionally mounted on a vertical linear guide having a linear measuring system for detecting the movement of the linkage arm in the vertical z-direction.
The advantage of such arrangements having two parallel pivot axes is the simple mechanical configuration of the low-mass linkage parts. These linkage parts are easy to manipulate when compared to apparatus having cartesian guides and make possible a simple manipulation, for example, for continuous contact scanning of profiles.
However, in order to be able to carry out high precision measurements with the above-mentioned linkage arm apparatus, it is necessary to precisely know the basic geometry of the apparatus, that is, the lengths R1, R2 of the linkage arms, the zero positions .phi.0 and .psi.0 of the angle measuring systems corresponding to the pivot axes and the diameter dT of the probe ball inserted for measurement. For this purpose, the apparatus must be calibrated not only once but each time when the above-mentioned primary dimensions change. This is, for example, always the case when a new probe is exchanged and inserted. Not only does the diameter of the probe ball change, as a rule, but also the effective length R2 of the second linkage arm.
Furthermore, it is also purposeful to detect the nonparallelism of the pivot axes with respect to each other and with respect to the linear guide on which the linkage arm is mounted. It is also purposeful to detect the elastic deformations of the individual linkage members caused by their own weight and by the actuating and measuring forces in the context of a calibration. The last-mentioned influences can cause systematic errors which become effective via the linkage system in a very complicated manner as spatial measurement and position errors of the probe.
For calibrating coordinate measuring apparatus, laser interferometers have been used in most cases up until now. With laser interferometers, the position of a reflector prism attached to the probe was determined when moving the probe in the measuring range of the coordinate measuring apparatus independently of the measuring systems of the coordinate measuring apparatus. In addition, the so-called ball-rod method is used wherein a rod is provided at both ends with ball joints. In this method, the rod of known length is attached to the measuring table of the coordinate measuring apparatus and to the probe of this apparatus and is pivoted in the measuring range. Calibrating methods of this kind with laser interferometers and ball rods are, for example, described in European patent publication 0,304,460 and U.S. Pat. Nos. 4,819,195 and 4,884,889. The known calibrating methods are relatively time intensive and require many expensive calibrating tools. These methods are therefore not suitable for making routine calibrations of a coordinate measuring apparatus, for example, after each probe exchange. Furthermore, interferometers are suitable only for calibrating coordinate measuring apparatus which make measurements in cartesian coordinates.